In the second half of the last century a great number of echographic systems have been developed, capable to obtain information from surrounding means and from human body, which are based on the use of elastic waves at ultrasonic frequency.
Presently, the performance limit of these systems is due to the devices capable to generate and detect ultrasonic waves. Thanks to the great development of microelectronics and digital signal processing, both the band and the sensitivity, and the cost of these systems as well, are substantially determined by these specialised devices, generally called ultrasonic transducers (UTs).
The majority of UTs are made by using piezoelectric ceramics. When ultrasounds are used for obtaining information from solid materials, it is sufficient the employment of the sole piezoceramic, since the acoustic impedance of the same is of the same magnitude order of that of solids. On the other hand, in most applications it is required generation and reception in fluids, and hence piezoceramic is insufficient because of the great impedance mismatching existing between the same and fluids and tissues of the human body
In order to improve the performances of UTs, two techniques have been developed: matching layers of suitable acoustic impedance, and composite ceramic. With the first technique, the low acoustic impedance is coupled to the much higher one of ceramic through one or more layers of suitable material and of thickness equal to a quarter of the wavelength; with the second technique, it is made an attempt to lower the acoustic impedance of piezoceramic by forming a composite made of this active material and an inert material having lower acoustic impedance (typically epoxy resin). These two techniques are nowadays simultaneously used, considerably increasing the complexity of these devices and consequently increasing costs and decreasing reliability. Also, the present multi-element piezoelectric transducers have strong limitations as to geometry, since the size of the single elements must be of the order of the wavelength (fractions of millimeter), and to electric wiring, since the number of elements is very large, up to some thousands in case of array multi-element transducers.
In order to solve these problems, the electrostatic effect is exploited, that is a valid alternative to the piezoelectric effect for making ultrasonic transducers. Electrostatic ultrasonic transducers, made of a thin metallised membrane (mylar) typically stretched over a metallic plate (also called rear plate or “backplate”), have been used since 1950 for emitting ultrasounds in air, while the first attempts of emission in water with devices of this kind were on 1972. These devices are based on the electrostatic attraction exerted on the membrane which is thus forced to flexurally vibrate when an alternate voltage is applied between it and the backplate; during reception, when the membrane is set in vibration by an acoustic wave, incident on it, the capacity modulation due to the membrane movement is used to detect the wave.
The resonance frequency of these devices is controlled by the membrane tensile stress, by its side size and by the thickness as well as the backplate surface roughness. Typically for emission in air, the resonance frequency is of the order of hundred of KHz, when the backplate surface is obtained through a turning or milling mechanical machining.
In order to increase the resonance frequency and to control its value, transducers have been developed which employ a silicon backplate, suitably doped to make it conductive, the surface of which presents a fine structure of micrometric holes having truncated pyramid shape, obtained through micromachining, i.e. through masking and chemical etching. With transducers of this type, known as “bulk micromachined ultrasonic transducers”, maximum frequencies of about 1 MHz for emission in water and bandwidths of about 80% are reached. However, the characteristics of these devices are strongly dependent on the tension applied to the membrane which may not be easily controlled.
It has been recently developed a new generation of micromachined silicon capacitive ultrasonic transducers known as “surface micromachined ultrasonic transducers” or also as Capacitive Micromachined Ultrasonic Transducers (CMUTs). CMUTs, and related processes of manufacturing through silicon micromachining technology, have been described, for instance, by X. Jin, I. Ladabaum, F. L. Degertekin, S. Calmes, and B. T. Khuri-Yakub in “Fabrication and characterization of surface micromachined capacitive ultrasonic immersion transducers”, J. Microelectromech. Syst., vol. 8(1), pp. 100-114, September 1998, by X. Jin, I. Ladabaum, and B. T. Khuri-Yakub in “The microfabrication of capacitive ultrasonic Transducers”, Journal of Microelectromechanical Systems, vol. 7 No 3, pp. 295-302, September 1998, by I. Ladabaum, X. Jin, H. T. Soh, A. Atalar and B. T. Khuri-Yakub in “Surface micromachined capacitive ultrasonic transducers”, IEEE Trans. Ultrason. Ferroelect. Freq. Contr., vol. 45, pp. 678-690, May 1998, by U.S. Pat. No. 5,870,351 to I. Ladabaum et al., by U.S. Pat. No. 5,894,452 to I. Ladabaum et al., and by R. A. Noble, R. J. Bozeat, T. J. Robertson, D. R. Billson and D. A. Hutchins in “Novel silicon nitride micromachined wide bandwidth ultrasonic transducers”, IEEE Ultrasonics Symposium isbn:0-7803-4095-7, 1998.
These transducers are made of a bidimensional array of electrostatic micro-cells, electrically connected in parallel so as to be driven in phase, obtained through surface micromachining. In order to obtain transducers capable to operate in the range 1-15 MHz, typical in many echographic applications for non-destructive tests and medical diagnostics, the micro-membrane lateral size of each cell is of the order of ten microns; moreover, in order to have a sufficient sensitivity, the number of cells necessary to make a typical element of a multi-element transducer is of the order of some thousands.
The process for manufacturing CMUT transducers is based on the use of silicon micromachining. In order to make the base structure of a CMUT transducer, that is an array of micro-cells each provided with a metallised membrane stretched over a fixed electrode (lower electrode), six thin film deposition and six photolithographic steps are generally employed.
The device is grown onto the oxidised surface of a silicon substrate. The lower electrodes of the micro-cells are obtained through photolithographic etching of a metallic layer deposited onto the oxide layer of the silicon substrate. The thus obtained electrodes are protected through a thin layer of silicon nitride that is generally deposited with PECVD techniques.
In order to obtain the micro-cell structure, a sacrificial layer (for example of chromium) is deposited, through evaporation, onto the silicon nitride layer. Through a new photolithographic step, the sacrificial layer is etched so as to form a set of small circular islands which will define the cavity underlying the membrane of the single micro-cells. A silicon nitride layer is then deposited on the whole surface of the substrate so as to cover the surface of the circular islands of sacrificial material. This layer will constitute the membranes of the single micro-cells.
In fact, these membranes are released through a wet etching of the sacrificial layer that acts through small holes, made through a dry etching with reactive ions, or RIE (Reactive Ion Etching) etching, through the same membranes, in other words through the silicon nitride layer covering the islands of sacrificial material.
FIG. 1 shows the image, obtained through a scanning electron microscope or SEM, of a section of a silicon nitride membrane suspended over a cavity. It should be noted the typical shape of the cavity that is extremely long with respect to the thickness.
The critical step of this technology is the indispensable closure of the holes made through the micro-membranes, necessary for emptying the cavities of the sacrificial material. Closure of these holes, even if not necessary from the functional point of view (emission and reception of acoustic waves), is indispensable, in practical applications, for preventing the same cavities from being filled with liquids and also wet gases with evident decay of performance.
To this end, it is used a subsequent deposition of silicon nitride of thickness such as to close the holes without, however, excessively penetrating under the active part of the membrane. The nitride layer that is deposited onto the membranes is afterwards removed in order not to alter the membrane thickness, that is a parameter strongly affecting the performance of the device.
For completing the device, a layer of aluminium is then deposited, that is subsequently etched through photolithography, so as to form the upper electrodes of the micro-membranes and the related electric interconnections. Finally, a thin layer of silicon nitride is deposited onto the device in order to passivate it and insulate the same from the external ambience.
FIG. 2 shows an image obtained through optical microscope of a portion of a finished device. Since nitride is transparent, there may be noted the micro-cavities 1 on which the membranes are suspended, the closed emptying holes 2, the electrodes 3 having radius lower than that of the membranes, and finally the electric interconnections 4.
However, conventional processes for manufacturing CMUT transducers, through micromachining, present some limitations.
First of all, the holes made onto the membrane surface, necessary for removing the sacrificial material, perturb the membrane uniformity.
Moreover, filling and sealing the holes, after releasing the membranes, are of difficult achievement. In particular, such step is certainly critical along the whole process for manufacturing CMUTs, and it has been often identified as possible cause of unsuccessful operation of the devices. Hole elimination, at least on the structural membrane in contact with the propagation environment, alone would produce evident advantages.
Furthermore, as also disclosed in literature, silicon nitride, of which the structural membrane is constituted, is intrinsically porous. The porosity of the nitride so far used in technological processes of CMUTs is to be investigated in the used deposition method. In fact, PECVD technique, although offering other advantages (low temperatures of deposition and possibility of varying with continuity the film mechanical characteristics), produces a porous nitride film. The attempts of solving such problem, through increasing the nitride thicknesses (by consequently reducing the membrane porosity), are not adequate, because they vary in a unacceptable way the electro-acoustic characteristics of the membranes.
Still, conventional processes for manufacturing CMUT transducers generally use seven lithographic masks. A so large number of masks involves a consequently long time for machining a silicon wafer. Moreover, the possibility of introducing errors in alignment is similarly high.
Finally, present technology provides the presence of transducer connection pads on the same surface of the active elements. Although from the point of view of simplicity this is the best solution, it is not so for the packaging problems. In fact, the best solution in this case provides the presence of the contacts in the device back part. In this regard, in literature CMUT devices have been described which use connection pads located on the back surface of the same device, but to this end techniques have been used for making deep trenches crossing the whole silicon wafer with related metallisation of the inner surfaces of the resulting holes.
Document US-A-2004/0085858, U.S. Pat. No. 6,958,255, discloses a surface micromechanical process for manufacturing one or more micromachined capacitive ultra-acoustic transducers, each one of which comprises one or more electrostatic micro-cells, each micro-cell comprising a membrane of conductive elastic material suspended over a conductive substrate, comprising the step of having a semi-finished product comprising a silicon wafer having a face covered by a first layer of elastic material.
Document FR-A-2721471 discloses a surface micromechanical process for manufacturing one or more micromachined ultrasonic transducers having a variable capacity, each one of which comprises one or more electrostatic micro-cells provided with a plurality of apertures, each micro-cell comprising a membrane of conductive elastic material suspended over a conductive substrate, comprising the step of having a semi-finished product comprising a silicon wafer having a face covered by a first layer of elastic material.
Document US-A-2003/0114760 discloses a conventional surface micromechanical process for manufacturing one or more micromachined capacitive ultra-acoustic transducers, further comprising, afterwards the CMUT formation, steps for providing an acoustically-damped region below the MUTs to substantially inhibit the propagation of acoustic waves in the substrate.
Document US-A-2001/0043029 refers to a conventional surface micromechanical process for manufacturing CMUTs having vibrating membranes separated from the silicon wafer and provided with conductive layers placed over the membranes.